This invention pertains to a method of improving the resistance to fatigue failure of coupling and connection joint mechanisms, specifically with respect to joint mechanisms operative to connect two mechanical elements which are required to rotate in unison and generally coaxially about a common axis. Even more specifically, the present invention provides improvements and finds substantial utility in the field of earth drilling equipment and specifically with respect to improving the fatigue failure resistance of threaded connections in drill collars and tool joints.
Fatigue failures typically occur as a result of gradual progressive growth of minor irregularities into major cracks. Minor cracks or irregularities typically originate in areas where the stress is concentrated, as for example at sharp notches, grooves or other surface discontinuities. Upon the application of a tension force, the minor cracks propagate and enlarge, and failure is the eventual result. In situations of cyclic variations in stress or cyclic stress reversals, where tension forces are periodically replaced by compression forces or tension forces of a different magnitude, the alternating stress magnitude more rapidly propagates the minor cracks to major detrimental proportions. Cyclic variations in stress or cyclic stress reversals will cause fatigue failure even when the average tensile stress applied is much less than the yield strength of the material.
All of the factors promoting fatigue failure are acutely present in earth drilling situations. All of the drill collars or pipes comprising the drill string, and all of the drilling tools connected to the drill string, are connected together at threaded coupling and connection joints. The threads, of course, define relatively sharp notches and grooves and are areas where stress is inherently concentrated. In many cases, the threaded ends of the drill collars and tools are of reduced size and therefore strength, leaving less material to inherently resist fatigue failure. All of the drill string elements except those within a few hundred feet of the drill bit are operated in a continuous state of tension due to the fact that the weight of the relatively lengthy drill string, even when buoyed by the drilling fluid, typically exceeds that weight required to obtain effective drill bit penetration and cutting rates. Only under extremely rare conditions will a well bore be cut and advanced along a straight axis because the geological formations will typically induce lateral deflection of various degrees in the drilling equipment as drilling progresses. As the drill string is pulled tangent to the inner curved wall of a well bore dogleg, bending is predominant in the coupling and connection joints. Rotation of the bent drill string creates cyclic stress reversals or stress alterations with an added compression force occurring at points on the drill string rotating adjacent the inner curve of the dogleg and with an added tension force occurring at diametrically opposite points on the drill string. The chemicals introduced into the drilling fluid or mud have a corrosive effect on the drilling equipment, and after a period of use may induce surface irregularities in the form of corrosion pitting and the like. These irregularities can also create areas of stress concentration.
The detrimental nature of the service-induced forces at play on earth drilling tools, and the necessity for improving the fatigue resistance for coupling and connection joints in earth drilling tools, have previously been recognized. The previous procedure for increasing fatigue resistance has been to cold work the roots of the threads, either by use of a hydraulic roller or a peening tool. The metal in the thread root is left in a compressed state that gives improved resistance to fatigue failure. The residual compressive stresses induced by cold working extend only to a relatively shallow depth below the surface of the thread roots, and the magnitude of the induced resistance to fatigue failure is similarly limited. From this and other background information, the objectives of the present invention are more readily appreciated.